When you redirect port 25 to port 500, you don't need to redirect port 500 to port 25 again. NAT works with connections and it will handle return packets for you.
For example try to direct port 23 to port 22 which ssh is listening
iptables -t nat -A PREROUTING -p tcp --dport 23 -j REDIRECT --to-port 22
Now you can connect that device via port 23
ssh foo@mysshserver -p 23
In your case you should add this rule
iptables -t nat -A PREROUTING -p tcp --dport 500 -j REDIRECT --to-port 25
Now all client are able to connect via port 500 to daemon listening port 25